This is probably all stuff you already know, but I thought I'd share my thoughts.
I buy and sell a lot of used equipment. I would not negotiate specific prices via email, it's annoying. Ask the person if he is flexible if you buy everything today. If yes, don't make an offer. Go meet face to face with cash in hand.
I would decide on the price you want to pay and take it in 50 dollar bills. If the person tries to negotiate for more money, give up a few of lighter pairs of weights (or what ever pair(s) you don't care about) as a trade off, instead of paying more.
From my experience, five fifty dollar bills look more enticing than a wad of tens and twenties totalling the same amount. Why, I don't know. It works the same with hundreds if you are buying in a higher price range.
FYI-25 cents a pound is the going rate for buying used weights in my market.